It is generally known in the art that the laminar-flow conditions of the boundary layer of a fluid flowing over the surface of a body can be influenced by various devices. It is further known that stabilizing the laminar-flow conditions of the boundary layer can reduce the resulting skin friction between the fluid and the body. This is especially pertinent, for example, in the field of aircraft construction, whereby the improvement of the laminar-flow of the boundary layer and the resulting lower skin friction can achieve potential fuel savings in the operation of the aircraft.
For these reasons, the use of surface suction through a porous or perforated surface for stabilizing the laminar-flow boundary layer of the fluid flowing over, or relative to, the surface has been widely studied for many decades. In the context of commercial aircraft, known laminar-flow control devices generally must operate with the best efficiency at only one condition, namely the cruise flight condition, and are thus designed primarily for this operating condition.
It has been preferred to apply surface suction through a purposely perforated surface, rather than a porous surface, due to the control of the surface characteristics that can be achieved by purposely forming the perforations, for example with desired sizes, patterns, and spacings. It is known to form such perforations by mechanical drilling, etching, electron beam boring, or laser beam boring. Typical conventional perforation designs provide perforations that are essentially small holes with circular plan or sectional shapes, with a diameter much smaller than the thickness of the boundary layer flowing over the surface. Typical diameters of the perforations or holes are conventionally in the range of 50 to 100 μm. Typical conventional spacings between adjacent perforations range from 200 to 5000 μm.
According to the prior art, the holes or perforations are typically provided in patterns that are regular and spatially repeating or similar (e.g. essentially translationally invariant) over wide portions of the surface. Examples of such patterns are checkerboard patterns, or patterns of linear rows of holes with essentially equal hole-to-hole spacing along each row and essentially equal row-to-row separation. The perforation density is generally held constant, for machining convenience and the like, but advantages of spatially varying porosity have been discussed. For example, U.S. Pat. No. 5,263,667 (Horstman) describes a rectilinear pattern of perforations with spatially varying perforation density, in an effort to achieve an essentially constant suction velocity in a region of varying external pressure.
The following U.S. patents are also generally related to the art of boundary layer control by suction: U.S. Pat. Nos. 5,884,873; 5,899,416; 6,050,523; 6,216,982; and 6,415,510.
All known perforation patterns described in the previous art are generic in the sense that they are not determined from, and do not reflect or contain, any information regarding the structure, form, flow conditions, or especially flow instabilities of the boundary layer that flows over the perforated surface. Since the abatement and elimination of such flow instabilities is a primary purpose of the laminar-flow control system, as recognized and developed by the present inventor, all previously existing or suggested perforation patterns of conventional surface suction systems yield either inefficient or dysfunctional laminar-flow control results. The basic reason for such inefficiency lies within the physics governing the boundary layer instabilities.
As known by those skilled in the art, boundary layer flows developing over a swept wing, a swept vertical stabilizer, or a swept horizontal fin of an aircraft have three velocity components and are thus called three-dimensional (3-D) boundary layer flows. While the laminar-turbulent flow transition in two-dimensional boundary layers is dominated by traveling waves known as Tollmien-Schlichting waves (TS waves), the three-dimensional boundary layers are high unstable to steady cross-flow vortices (CF vortices), which dominate the laminar-turbulent flow transition process in the three-dimensional flow context.
Experiments have shown that suction through a conventional perforation pattern in a 3-D boundary layer has two opposing effects, namely one of stabilization due to a change in the mean velocity profile, and one of destabilization due to the excitation of steady cross-flow vortices by variations and inhomogeneities in the suction distribution. In this regard see H. Bippes (1999), “Basic experiments on transition in 3D boundary-layers dominated by crossflow instability”, Progress in Aerospace Sciences 35: 363–412 and D. Arnal, A. Seraudie, J. P. Archambaud, “Influence of surface roughness and of suction on the receptivity of a swept-wing boundary layer”, Laminar-Turbulent Transition, IUTAM Symposium, Sedona Ariz., Sep. 13–17, 1999, Springer, 2000. It has also been observed that there is a clear limit to the amount of suction that can be applied to 3-D boundary layers, beyond which the flow in the vicinity of each hole becomes sufficiently distorted to cause the flow to undergo an immediate and irrecoverable transition to turbulence. This effect is called “oversuction”. In this regard, see P. Wassermann and M. Kloker, “DNS-investigations of the development and control of cross-flow vortices in a 3-D boundary-layer flow”, Laminar-Turbulent Transition, IUTAM Symposium, Sedona Ariz., Sep. 13–17, 1999, Springer, 2000.
At any suction level, the hole pattern has a dominant influence on non-uniformities in the suction distribution. For values of suction strength below the “oversuction” level, the present inventor has previously described a formulation for determining the wavenumber components of the surface hole distribution that are most efficient in stimulating unstable boundary-layer modes. See F. P. Bertolotti (2000), “Receptivity of three-dimensional boundary-layers to localized wall roughness and suction”, Phys. Fluids, Vol. 12, Number 7, pg. 1799–1809). In most conventional cases, the hole-to-hole spacing is smaller than the smallest wavelength of amplified disturbances. Theoretical results assuming a perfectly homogeneous wall-suction distribution in space show that both the TS waves and the CF vortices are strongly stabilized by suction, in contradiction to the above mentioned experimental findings. The cause has been traced to variations, or inhomogeneities, in the actual suction distribution in the experiments, as a result of various phenomena that introduce unwanted and harmful variations in the suction distribution pattern. Namely, it has now been considered by the present inventor, that the following phenomena introduce such unwanted and harmful variations in the suction distribution pattern.                a) Unavoidable boring or machining inaccuracies and tolerances, to which the overall laminar-flow control efficiency is highly sensitive;        b) Clogging of perforations by contamination or particulate matter during operation;        c) Blockage of perforations by the structure supporting the perforated skin;        d) Suction inhomogeneity within the internal suction chambers applying suction to the perforated skin;        e) Chordwise variations in external pressure; and        f) Flow distortions in the vicinity of a perforation due to large suction velocities.        
Among the above phenomena, the phenomena identified as a), b) and c) introduce variations in the perforation geometry, while d), e) and f) introduce variations in the suction strength and flow conditions. All of these phenomena produce harmful variations in the suction pattern. Furthermore, these phenomena can have interactive effects with one another. For example, when only a single suction plenum, or only a few suction plenums are used below the perforated skin, the phenomenon e) strongly affects the pressure-drop across the skin and may cause “oversuction” to occur at some locations, resulting in flow distortions according to phenomenon f).